Systems for determining positions along a direction of advance are required in many fields of application in which the position of an object that is mobile along a direction of advance needs to be determined, e.g. the position of a machine component on a linear axle, such as drives or swivel arms. The positions sensed in this case can be used as position values for measurement purposes, or else for positioning components using a drive with a position control loop. Such position finding systems are accordingly found in devices such as coordinate measuring machines (CMM), geodetic devices, robot arms or hydraulic actuators. Such a system normally has a pattern and a sensor, with either the pattern or the sensor being connected to the mobile object and the pattern and the sensor being mobile relative to one another along the direction of advance. The sensor scans the pattern and produces a scan signal that is dependent on the scanned pattern section. The pattern is structured such that the scan signal changes upon a change in the relative position of the sensor in relation to the pattern and hence the position that is to be determined for the object. A control and evaluation unit of the system ascertains the sought position from the scan signal using stored information about the pattern. By way of example, the pattern for incremental position finding has pattern elements of the same type that are arranged at the same respective distance from one another in the direction of advance. The sensor scans one pattern element in each case. The information stored in the control and evaluation unit is the distance between the pattern elements, so that, on the basis of a defined zero position for the sensor and the pattern, it is possible to determine the position from the number of the scan signals of the same type that are scanned for a relative movement by the sensor and pattern. A disadvantage of systems with incremental position finding is that, when the system is restarted, it is necessary to start from a zero or reference position again each time.
By contrast, in the case of systems with absolute position finding, respective individual pattern elements or successions of pattern elements produce unique and distinguishable scan signals. This allows a unique position to be associated with a respective relative position for the sensor and the pattern immediately, i.e. without approaching a reference position.
Ideally, sensor and pattern change their relative position in relation to one another only in the direction of advance, i.e. their movement relative to one another has only one degree of freedom (DoF). In practice, however, undesirable deviations from this ideal situation frequently arise, i.e. the position of the sensor in relation to the pattern varies unintentionally often in at least one further spatial direction, i.e. in respect of at least one further degree of freedom of movement, e.g. in the form of an offset, a skew or a tilt. In this case, the offset is either constant or, on account of the direction of advance and the longitudinal axis of the pattern not being exactly parallel to one another, dependent on the relative position. In addition, skew or tilts in the sensor can arise, i.e. unintentional rotation of the sensor about at least one of its axes. Such discrepancies from the ideal situation can adversely affect measurement accuracy. Keeping such deviations as small as possible, i.e. observing the ideal position as accurately as possible, or compensating for deviations requires a high level of design complexity and/or complex calibration methods, particularly when there is a large region of movement in the direction of advance.
As an alternative, U.S. Pat. No. 6,333,511 discloses a system for determining positions of an object along a direction of advance, which is furthermore intended to be able to be used to identify a slight movement by the object or the sensor perpendicular to the direction of advance or is intended to allow flush orientation of the sensor with the direction of advance. The system is based on optoelectronic, incremental position finding and has an alternately opaque and transparent strip pattern along the direction of advance, which is scanned by a sensor having an array of detectors using trans-illumination methods. The, preferably uniform, distance of the strips in relation to one another is known and stored in a control and evaluation unit. The strip pattern and the sensor are attuned to one another such that, in each relative situation of the pattern in relation to the sensor, there is ever only no more than one edge of a strip, i.e. a boundary between an opaque strip and a transparent strip, scanned by a detector. Depending on the relative situation of an opaque strip in relation to the detector (and hence the pattern or the position of a mobile object connected to the pattern), the detector senses different amounts of light, so that the light-dependent intensity of a detector signal produced by a detector in each case is dependent on the relative position.
As a result, the sought position can be ascertained incrementally using the detector signal intensity of an individual, suitably selected detector. A suitable detector is one that scans an edge of a strip and, in so doing, neither senses the minimum possible light intensity (in the case of complete dimming as a result of an opaque strip) nor senses the maximum possible light intensity (in the case of complete illumination as a result of a transparent strip). To increase precision, the final position is computed by means of averaging from many position values ascertained by means of individual detectors. Determination of movements (see above) by the object or the sensor perpendicular to the direction of advance or a flush orientation of the sensor in the direction of advance is made possible according to U.S. Pat. No. 6,333,511 by the use of a pattern of v-shaped strips or an additional pattern with strips that are oriented parallel to the direction of advance. A disadvantage of the system according to U.S. Pat. No. 6,333,511 is that firstly absolute position finding is not possible, secondly it is not possible to determine further deviations from an ideal position for the sensor relative to the pattern other than a slight movement by the object or the sensor perpendicular to the direction of advance and a flush orientation of the sensor, and thirdly orientation of the detector array along the direction of advance, i.e. scanning in the direction of advance instead of orthogonally in relation thereto, is admittedly possible but is associated with losses of precision. Absolute position finding is possible in the case of the system disclosed in U.S. Pat. No. 6,333,511 only by using additional position finding means or an additional strip pattern that extends parallel to the first pattern in the direction of advance. The additional means or the additional strip pattern can be used to determine an absolute position, as a result of which the incremental position finding described can be located absolutely, i.e. only the incremental pattern and the additional position finding means or the additional pattern can be used to ultimately indicate an absolute position. A disadvantage in this case is that the absolute position can be determined only relatively coarsely.
EP 0042179 likewise discloses a system for determining positions along a direction of advance, which system allows determination of deviations from an ideal position using a pattern having v-shaped strips. In this case too, a position determined incrementally using a pattern is located absolutely by means of a further separate, absolute pattern extending parallel to the first pattern in the direction of advance, with the absolute pattern of EP 0042179 coding coarse positions using a Gray code.